Add default_visibility for soong modules

So that we don't get surprise usages.

Bug: 348717861
Test: m nothing
Change-Id: I47319a727092c1bd936cca88f5713ee977a48b48

cmd/zip2zip/Android.bp

@@ -27,4 +27,6 @@ blueprint_go_binary {
         "zip2zip.go",
     ],
     testSrcs: ["zip2zip_test.go"],
+    // Used by genrules
+    visibility: ["//visibility:public"],
 }